The invention relates to a method and a device for testing coins, in which the image of the coin is detected using an image sensor.
In the process of recognising the embossing on coins there is a known way of using photographic image sensors which usually have pixels arranged in columns and lines. Here it is necessary to determine the correct moment of the recording of the moved objects i.e. the coin (here the term “coin” is used to denote also tokens or counters or the like). One problem is detecting the coin in its unrestricted course of movement and monitoring the run of the coins only with the image recording system present or the image sensor. During the dynamic behaviour of a coin as it passes an image sensor and the production of an image by the optical image sensor, exact analysis of the times of passing through the imaging area and the recording speed of the sensor must be made. It is crucial to determine the correct value for the exposure time and the moment of the recording. Although it is possible in special cases, it is not to be expected that the coin will move through the recording area of the image sensor at a constant speed and certainly will not stay there for a short time at all. There are four time areas to be examined, namely the run-in of the coin into the sensor area, the recording/exposure time, the data transfer or transmission for processing the image in an evaluation device and the image processing, analysis and evaluation. Whilst for the fourth time period no fixed time value is given, apart from the maximum time of the entire process, the first three time periods are extremely time-critical.
Run-in times for selected coins were determined, the run-in time being determined as the time from the appearance of the edge of the coin in the recording area of the image sensor up to complete detection of the coin by this recording area. The measured run-in time was between 4.5 and 9.9 ms and that of 0.01, 0.10, 1 and 2 Euro coins was respectively 4.9, 5.9, 6.99 and 7.71 ms. The aimed-at diameter range of the coins to be measured is between 15 mm and 33 mm. Measurements in real coin checkers produced a maximum coin speed of 3 m/s. With the given time conditions it is inconceivable to follow the run of the coin up to the correct recording position by cyclical scanning of the image. Moreover in such a case the data transfer times from the image sensor to the operating processors is much too long.